Rescuers are attempting to approach two burning ships in the Black Sea near Crimea's Kerch Strait after the cargo vessels collided. One of them, a liquefied natural gas carrier, has exploded.
Unconfirmed Russian media reports say 14 people have been killed, with an unknown number still in need of rescue.
Russian rescue authorities say the two ships were alongside each other transferring fuel when a fire started. The flames quickly spread to both ships, setting off an explosion aboard one of them.
The ships involved were the Kandy, with a crew of 17, and the Maestro, with 14 on board.
AFP reports "no signal from either one of the two captains" had been received.
Reports state 12 people have so far been rescued.
"Eight people remain missing. The search and rescue operation continues," the Maritime Rescue co-ordination Center said in a statement. "There is a threat of a gas explosion,
Russian news service TASS reports rescuers as saying the two Tanzania registered ships were 'illegally anchored'.
The incident has occurred in neutral waters outside the contested Kerch Strait, linking the Sea of Azov with the Black Sea. Russia has moved to seize the international waterway following its annexation of the adjoining Crimea Peninsula in 2014.
Russian media is reporting firefighting vessels are currently attempting to douse the flames on both ships.